Color space transformations between imaging devices are dependent on the illuminants used. The mismatch of white points is a frequently encountered problem. It happens in situations where the measuring and viewing of an object are under different illuminants, the original and reproduction use different illuminants, and the different substrates are under the same illuminant. Normally, this problem is dealt with by chromatic adaptation in which the illumination difference is treated with an appearance transform.
Chromatic adaptation deals with the visual sensitivity regulation of the color vision such as the selective, yet independent, changes in responsivity of cone photoreceptors with respect to surround and stimuli. For example, a photographic scene viewed under different illuminants (e.g., tungsten lamp versus daylight D65) looks pretty much the same in spite of the fact that the reflected light is very different under these two illuminants. This is because our eyes have adapted under each condition to discount the illuminant difference. This is known as the âcolor constancyâ of human vision. To understand and, therefore, to predict the color appearance of objects under a different illuminant is the single most important appearance-matching objective. It has many industry applications, such as the cross-rendering between different media. There are two main approaches; one is the chromatic adaptation employing visual evaluations, and the other one is a mathematic computation of the color constancy (or white conversion). The main goal of the computational color constancy is to predict the object's surface reflectance. If the surface spectrum is known, the correct tristimulus can be computed under any adapted illumination. The computational color constancy will be presented in Chapter 12. This chapter reviews methods of chromatic adaptation with an emphasis on the mathematical formulation. Several models, ranging from the von Kries hypothesis to the Retinex theory, are presented.
4.1 Von Kries Hypothesis
There are numerous methods developed for the chromatic adaptation that can be found in many color textbooks. The most important one is the von Kries hypothesis, which states that the individual components present in the organ of vision are completely independent of one another and each is adapted exclusively according to its own function.
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